In-plane switching (IPS) is a leading liquid crystal display (LCD) technology, which has as one of its main features the ability to align liquid crystals horizontally to increase their viewing angle while allowing the transmittance of the LCD to be changed. Due to this, IPS technology was unanimously adopted at its introduction by the LCD industry and was one of the first refinements to LCDs, to be incorporated. As a result, wide viewing angles and fast response characteristics were able to be achieved, thus overcoming the two main drawbacks of standard twisted nematic (TN) based LCDs.
After its introduction in 1996, IPS LCD technology evolved through several generations, beginning with IPS and advancing on to Super IPS, Advanced Super IPS (AS-IPS), and finally to IPS-Pro. In particular, Super IPS LCD technology was introduced to overcome color shift problems of the original IPS displays that were apparent at wide viewing angles. AS-IPS allowed opaque electrodes to be replaced with transparent electrodes, which considerably reduced the amount of power that is required for the backlight of an AS-IPS LCD to operate. In addition, AS-IPS also provided smoother pixels, which yielded a cleaner, crisper, and more continuous image at all viewing angles. Finally, because IPS-Pro based LCDs are highly complex, they are expensive to produce, and as such, they are used only in applications where image clarity and precision is considered critical, such as in the medical field, including surgical applications, as well as in advanced engineering and science applications.
In an IPS liquid crystal (LC) cell, the molecules of the LC are aligned horizontally with an angle of about 6˜12 degrees with respect to the direction of the LCD electrodes. As such, the LC molecules are kept parallel to the electrode pair and the glass substrate of the LCD screen. Thus, when the IPS LC cell is placed between a pair of polarizers that are crossed at 90 degrees with respect to each other, such that one of the polarizers is aligned with the average LC molecule orientation in the absence of an electric field, the IPS LC cell appears as a black image. In this state, the polarized light passes through the cell without interruption from the LC molecules and is blocked by the front polarizer. To produce an image, a voltage is applied across the electrodes of the IPS cell to form an electric field, referred to as a lateral electric field, which is applied between each end of the liquid crystal molecules. The application of the lateral electric field causes the nematic LC molecules to be reoriented or aligned at an angle between the electrodes, normally at a 45-degree angle between the pair of crossed polarizers. Because the liquid crystal molecules are weakly anchored to the lower glass substrate of the IPS LC cell, they move more freely into the desired alignment upon application of the electric field. In addition, because there is no twisted structure in an IPS LC cell, as in a twisted nematic (TN) cell, the applied electric field causes the LC molecules of the IPS LC cell to be simply switched between dark and bright states in the plane with a fast response or switching speed. Moreover, IPS considerably improves viewing angles of TFT (thin film transistor) based LCDs as compared to TN (twisted nematic) LCDs due to the characteristic symmetrical optical retardation of TNs that occurs at all viewing angles.
IPS technology produces LCDs with high optical contrast between bright and dark images, while allowing wide viewing angles because the blockage of light transmission is complete at the field-off state, allowing the viewer to see black from all viewing angles. The optical transmittance of the aligned nematic LC layer between the polarizers crossed at 90 degrees with respect to each other can be given as T=sin2(2φ(V)) sin2(πdΔn(V)/λ). As such, φ(V) is the voltage-dependent azimuthal component of the angle between the LC optic axis and the transmission axes of the crossed polarizers, πdΔn(V) is voltage-dependent retardation of the LC layer (where d is the thickness of LC layer, Δn is the birefringence value of LC layer), and λ is the wavelength of incident light. With an applied voltage, the homogeneously-aligned LC molecules are switched in a direction parallel to substrates, making conditions of φ(V)=π/4 and πdΔn(V)/λ=π/2 to maximize the light transmittance, that is, T=1. The switching times of turn-on (rise, τon) and turn-off (decay, τoff) can be described by the following equations: τon=(γ1d2/πKeff)(1/[(V/Vth)2−1)] and τoff=(γ1d2/πKeff), where γ1 is the rotational viscosity of LC molecule, Keff is the effective elastic constant (for IPS, the Keff is equal to K22), and V and Vth are the applied voltage and threshold voltage, respectively. However, due to the slow decay time, current IPS LCDs are unable to display moving images with high quality.
Therefore, there is a need for an IPS LCD that combines a small amount of polymer network in a liquid crystal to stabilize or otherwise modify the reorientation of the LC director in response to an applied electric field. In addition, there is a need for an IPS LCD that provides a surface-localized polymer in an LC that enables fast switching of LC molecules, so as to improve dynamic response times, allowing the SS-IPS LCD to display moving images with high quality.